I've heard similar anecdotes about the VM being a mess. Where Adobe seems to have royally screwed up is creating an unambiguous specification for SWF format, a

I've heard similar anecdotes about the VM being a mess. Where Adobe seems to have royally screwed up is creating an unambiguous specification for SWF format, and also disallowing other implementations of SWF. Instead, they become beholden to a codebase that no one could make heads or tails of, with no way of creating a clean rewrite that was guaranteed to replicate the behavior. In designing Wasm we were very conscious of this, which is why Wasm has a very, very explicit specification that is simple with only the minimum non-determinism (some NaN issues) and no UB.